1. Field of the Invention
The present invention relates to a braking pressure generator for a vehicular hydraulic brake system, wherein a booster piston which is slidable by hydraulic auxiliary pressure is displaceable in the actuating direction. The booster piston is designed as a stepped piston with a smaller-diameter portion close to the pedal, and the booster piston includes a blind-end bore in which a master cylinder piston is sealedly guided. The smaller-diameter portion of the booster piston confines an annular chamber diminishing upon brake application. A valve is connected to the annular chamber and is controllable by the pressure in the pressure chamber of the hydraulic power booster, the valve closing a connection between the annular chamber and an unpressurized supply reservoir.
2. Brief Description of the Prior Art
A braking pressure generator having the foregoing features is described in German patent application No. P 34 08 873.3 (corresponding to U.S. Pat. No. 4,649,707 issued on Mar. 17, 1987). The braking pressure generator according to the application comprises a pedal-actuatable piston rod arranged coaxially to a booster piston. Parallel to the booster piston a so-called brake valve is disposed which is connected by way of a lever arrangement with the pedal-actuatable push rod. The face of the booster piston close to the pedal limits a pressure chamber which, depending on the position of the brake valve, is connected with an unpressurized supply reservoir or, alternatively, with an auxiliary energy source.
The booster piston is substantially designed as a stepped piston with an annular chamber being created between the piston step with the smaller diameter and the housing of the braking pressure generator, with the volume of the annular chamber diminishing upon brake application. The annular chamber normally (with the brake in released condition and the auxiliary pressure source not operative ) communicates by way of a two-way/two-position directional control valve with an unpressurized supply reservoir. The two-way/two-position directional control valve can be controlled hydraulically, while the pressure from the pressure chamber of the hydraulic power booster can be fed to the hydraulic control drive.
If an actuating force is applied to the brake pedal in the brake system described, the piston rod connected to the brake pedal is displaced relative to the booster piston so as to actuate the lever mechanism, which at first causes a hydraulic connection between the pressure chamber of the hydraulic power booster and the unpressurized supply reservoir to be interrupted. When the force at the brake pedal is increased, the brake valve eventually assumes a position in which the pressure chamber of the hydraulic power booster is supplied with hydraulic pressure from an auxiliary pressure source. Once a predetermined level is exceeded, the pressure then prevailing in the pressure chamber of the hydraulic power booster is finally capable of displacing the booster piston in the actuating direction, which in turn causes the tandem master cylinder connected downstream of the hydraulic power booster to be pressurized. Thus, actuation of the brake circuits connected to the tandem master cylinder is effected.
As described, during a brake actuation the volume of the annular chamber between the booster piston and the housing of the braking pressure generator diminishes. The pressure prevailing in the pressure chamber of the hydraulic power booster, apart from displacing the booster piston, also causes the two-way/two-position directional control valve connected to the annular chamber to be switched into a locking position so that the annular chamber is separated from the unpressurized supply reservoir. The volume of pressure fluid enclosed in the annular chamber in this operating condition is additionally supplied, upon further displacement of the booster piston, by way of respective sealing collars to the working chamber of the master cylinder which is connected downstream of the hydraulic power booster, in such a way as to result in a relative displacement between the booster piston and the master cylinder piston, with the master cylinder piston traveling a longer distance than the booster piston.
As is known, no auxiliary pressure can be built up in the pressure chamber of the hydraulic power booster in case of a failure of the auxiliary pressure source. Accordingly, the two-way/two-position directional control valve remains in its idle position in which the annular chamber is permanently connected with the unpressurized supply reservoir. In case of a displacement of the booster piston, which is now effected exclusively by mechanical force acting on the brake pedal, the volume in the annular chamber can relieve pressure to the unpressurized supply reservoir, without any additional forces counteracting the brake application.
In a similiar fail condition, therefore, only the considerably smaller area of the master cylinder piston is effective, although it is ensured by appropriate dimensioning that the required minimum braking effect is achieved.
It is a disadvantage in the brake system described that when the brake is applied while the auxiliary pressure source is intact, the volume enclosed in the annular chamber is supplied by way of an additional sealing collar into the working chamber of the master cylinder.
Therefore, it is the object of the present invention to simplify the design of a braking pressure generator of the type described.